1. Field of the Invention
The present invention relates to a color electrophotographic device; and, in particular, it relates to a color electrophotographic device comprising developing units using a developing powder composed of two components.
2. Description of Related Art
A color electrophotographic device, such as a color laser printer utilizing electrophotographic processes, comprises a photosensitive body on which there is formed electrostatic latent images corresponding to respective print data separated in accordance with colors of an original print. A plurality of developing units are also provided, each including a developing powder container accommodating a two component developing powder formed by mixing carriers and toners, and a developing powder agitating means is used for agitating the developing powder. A toner supplying means supplies the toners into the developing powder container, and a developing unit selecting and activating unit selects one of the developing units depending on a color to be developed and activates the same to develope an electrostatic latent image on the photosensitive body. A control unit provided for overall control of the above-mentioned operations. The toner consumed for the developing operation is supplied into the developing powder container from the toner supplying means.
A major use of a color laser printer is directed to the printing of graphic data. In particular, the frequency of dealing with print data having higher blackened rate, i. e., a high rate of colored dot printing, in comparison with that of text data by the color laser printer is increasing. When an electrostatic latent image of print data having a high blackened rate is developed, the amount of toner consumed in the developing unit increases, which causes a decrease in toner density in the developing powder and an increase frequency of toner supply in the developing unit.
Now, the two component developing powder formed by mixing carriers and toners is explained. Through mixing and agitating of the two component developing powder, the toners are frictionally charged and their deposition on the carriers and the electrostatic latent images is controlled. Accordingly, when the mixing and agitating of the two component developing powder is insufficient, the toners are not charged to a predetermined level, with the result that a desirable developing characteristic can not be achieved. Further, since the toner density is determined by detecting the permiability of the two component developing powder, which varies depending on the toner amount (mixture ratio of toner) deposited on the magnetic carriers via the frictional charge thereof, as a magnitude of inductance of a coil disposed adjacent to the two component developing powder, a correct toner density (a mixture ratio) can not be determined if a predetermined frictional charge is not induced on the toners. Accordingly, a sufficient mixing and agitating of the two component developing powder is indispensable.
When toners are supplied into a developing unit, a shortage in frictional charge of the two component developing powder is caused; however, in the case of a developing unit having a large storage capacity for the developing powder, an amount of the toners supplied in one time is relatively small with respect to the amount of toners under use, so that the influence of the shortage of charges due to the toner supply is insignificant. On the other hand, in case of a small size developing unit having a small storage capacity for the developing powder, the influence of the shortage of charges due to the toner supply is significant.
In particular, in a small size color laser printer necessitating four developing units, it is difficult to enlarge respective developing powder containers, with the result that, after a toner supply in association with print data having a high blackened rate, a shortage in toner charge is induced, which likely causes a defective developement. Further for maintaining a sufficient mixing and agitating of the developing powder, it has been necessary to interrupt frequently the printing operation for the mixing and agitating operation.